In mammals, behavioral and physiological processes display 24-hr rhythms that are controlled by circadian oscillators located in the hypothalamic suprachiasmatic nucleus (SCN). The SCN acts as a master pacemaker at the top of a hierarchy of circadian oscillators distributed throughout the body. Although the SCN is a relatively small nucleus in the brain containing about 10,000 neurons on each side, it is composed of many cell types. Two major classes of neuropeptide-containing neurons, VIP (vasoactive intestinal polypeptide) and AVP (arginine vasopressin), are enriched in the ?core? and the ?shell? regions of the SCN, respectively. It is known that the VIP and AVP neurons can subserve different functions, however, it has not been possible to study genetically identified cell types in the SCN in real time at the cellular level. We have developed a new generation of bioluminescent circadian reporter mice that are Cre-lox recombination dependent. Using cell-type-specific Cre drivers, these Cre-lox dependent reporters can be activated in restricted and genetically defined cell populations so that circadian properties of these cells can be studied separately from other cell types. In this proposal, we will analyze the cell-type specific circadian properties of VIP and AVP neurons within the SCN neuronal network by using a ColorSwitch PER2::LUCIFERASE reporter that has a click beetle red (CBR) luciferase fused to PER2 that switches to a click beetle green (CBG) luciferase upon Cre-lox recombination. With the cell-type restricted reporter, we can study for the first time the circadian properties of each of these neuropeptide classes of neurons in the SCN. Here we will study the role of VIP and AVP neurons in controlling circadian behavioral phenotypes using both loss-of-function and gain-of-function circadian mutations in these cell classes. We will also use optogenetic control of VIP and AVP neurons to analyze the dynamics of resetting within the SCN neuronal network. Finally, we will use single-cell RNA-seq of SCN cells to classify SCN cell types and in labeled VIP and AVP neurons in order to determine the molecular signatures and pathways characteristic of these two cell types. Together, these experiments will provide critical new information on the circadian properties and dynamics of the SCN in order to promote our understanding the circadian system in mammals, which is critical for understanding how circadian disruption in humans contributes to morbidity associated with neurological disorders, cognition, mental health, obesity, type 2 diabetes and cancer.